Place yolks, vanilla and sugar in a bowl that will rest easily on the top of a saucepan. Whisk until the mixture is thick and pale.
Place the bowl over the saucepan of gently simmering water (the bowl must not touch the water). Add Marsala to the yolk mix and, using an electric whisk, beat continuously until the mixture forms soft peaks (about 5-8 minutes). The mixture may be served warm, room temperature or chilled but do not leave in the fridge for longer than an hour.
In a mixing bowl, combine the berries. Spoon the zabaglione into four serving bowls or glasses along with the mixed berries. Garnish with almond biscotti and serve.